Jack Straw today became the first British minister to raise questions about the handling of the release of the man convicted of the Lockerbie bombing, Abdelbaset al-Megrahi.

As the Tories accused the Scottish government of giving a misleading account of the release, the justice secretary indicated he would have taken a different approach to that of his Scottish counterpart, Kenny MacAskill. Speaking at the Guardian's offices, where he was attending a seminar on House of Lords reform, Straw reiterated Gordon Brown's position that it would be wrong for a UK minister to offer an opinion on a decision taken in Edinburgh.

But he questioned whether it was right for MacAskill to visit Megrahi in prison as he decided whether to release him. Megrahi, who has terminal prostate cancer, was released on compassionate grounds last Thursday. Asked whether it was right for MacAskill to visit him in Greenock prison, Straw said: "That was his decision. If you are asking me if I have ever visited a prisoner in jail who has applied for compassionate release, the answer is no."

MacAskill said he was obliged to visit Megrahi in prison as he considered whether to transfer him to Libya under a prison transfer agreement negotiated by the British and Libyan governments. The Scottish justice secretary released him on compassionate grounds, after saying the British government had failed to answer questions about the prisoner agreement.

Straw said he had answered questions about the agreement from Scotland, pointing out that he had told the Scottish first minister, Alex Salmond, that prisoners could make representations in writing.

The remarks by the justice secretary were seized on by the Labour party in Scotland, which has condemned the release of Megrahi. Iain Gray, the Scottish Labour leader, said: "Alex Salmond must now publish immediately the advice that Kenny MacAskill acted on that led him to visit Megrahi in prison. Kenny MacAskill has not only misled parliament but the public at large."

The Scottish Tories also piled the pressure on MacAskill as they accused him of having given MSPs the inaccurate impression on Monday that Strathclyde police had warned against keeping Megrahi at home in Scotland because it would have caused "severe" security problems. Strathclyde police said they had advised MacAskill on policing levels needed to guard Megrahi but had made no judgment on whether that would be severe.